Apparatus and method for detecting dangerous conditions in power equipment

ABSTRACT

Woodworking machines are disclosed having electrically conductive cutting tools adapted to cut workpieces. The machines include a contact detection system capacitively coupled to the cutting tool, and adapted to detect contact between a person and the cutting tool. The machines also include a reaction system configured to cause one or more predetermined actions to take place upon detection of contact between a person and the cutting tool by the contact detection system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.09/929,221, filed Aug. 13, 2001, issuing as U.S. Pat. No. 7,284,467 onOct. 23, 2007, which in turn claims the benefit of and priority from thefollowing U.S. Provisional Patent Applications: 60/255,056, 60/255,057,60/255,058, 60/255,059, 60/255,089, 60/255,094, 60/255,169, 60/255,170,60/255,200, 60/255,201, 60/255,206, 60/255,210, 60/225,211, and60/255,212, all filed Aug. 14, 2000.

FIELD

The present invention relates to safety systems, and more particularlyto a high-speed safety system for use on power equipment.

BACKGROUND

Beginning with the industrial revolution and continuing to the present,mechanized equipment has allowed workers to produce goods with greaterspeed and less effort than possible with manually-powered tools.Unfortunately, the power and high operating speeds of mechanizedequipment creates a risk for those operating such machinery. Each yearthousands of people are maimed or killed by accidents involving powerequipment.

As might be expected, many systems have been developed to minimize therisk of injury when using power equipment. Probably the most commonsafety feature is a guard that physically blocks an operator from makingcontact with dangerous components of machinery, such as belts, shafts orblades. In many cases, guards are effective to reduce the risk ofinjury, however, there are many instances where the nature of theoperations to be performed precludes using a guard that completelyblocks access to hazardous machine parts.

Various systems have been proposed to prevent accidental injury whereguards cannot effectively be employed. For instance, U.S. Pat. Nos.3,953,770, 4,075,961, 4,470,046, 4,532,501 and 5,212,621, thedisclosures of which are incorporated herein by reference, discloseradio-frequency safety systems which utilize radio-frequency signals todetect the presence of a user's hand in a dangerous area of the machineand thereupon prevent or interrupt operation of the machine.

U.S. Pat. Nos. 4,959,909, 5,025,175, 5,122,091, 5,198,702, 5,201,684,5,272,946, and 5,510,685 disclose safety systems for use withmeat-skinning equipment, and are incorporated herein by reference. Thesesystems interrupt or reverse power to the motor, or disengage a clutch,upon contact with a user's hand by any dangerous portion of the machine.Typically, contact between the user and the machine is detected bymonitoring for electrical contact between a fine wire mesh in a gloveworn by the user and some metal component in the dangerous area of themachine. Although such systems are suitable for use with meat skinningmachines, they are relatively slow to stop the motion of the cuttingelement because they rely on the operation of solenoids or must overcomethe inertia of the motor. However, because these systems operate atrelatively low speeds, the blade does not need to be stopped rapidly toprevent serious injury to the user.

U.S. Pat. Nos. 3,785,230 and 4,026,177, the disclosures of which areherein incorporated by reference, disclose a safety system for use oncircular saws to stop the blade when a user's hand approaches the blade.The system uses the blade as an antenna in an electromagnetic proximitydetector to detect the approach of a user's hand prior to actual contactwith the blade. Upon detection of a user's hand, the system engages abrake using a standard solenoid. Unfortunately, such a system is proneto false triggers and is relatively slow acting because of the solenoid.

U.S. Pat. No. 4,117,752, which is herein incorporated by reference,discloses a similar braking system for use with a band saw, where thebrake is triggered by actual contact between the user's hand and theblade. However, the system described for detecting blade contact doesnot appear to be functional to accurately and reliably detect contact.Furthermore, the system relies on standard electromagnetic brakesoperating off of line voltage to stop the blade and pulleys of the bandsaw. It is believed that such brakes would take 50 ms-1 s to stop theblade. Therefore, the system is too slow to stop the blade quicklyenough to avoid serious injury.

None of these existing systems have operated with sufficient speedand/or reliability to prevent serious injury with many types of commonlyused power tools. Although proximity-type sensors can be used with someequipment to increase the time available to stop the moving pieces, inmany cases the user's hands must be brought into relatively closeproximity to the cutting element in the normal course of operation. Forexample, many types of woodworking equipment require that the user'shands pass relatively close to the cutting tools. As a result, existingproximity-type sensors, which are relatively imprecise, have not proveneffective with this type of equipment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a machine with a fast-actingsafety system according to the present invention.

FIG. 2 is a schematic diagram of an exemplary safety system in thecontext of a machine having a circular blade.

FIG. 3 is a schematic side elevation of an exemplary embodimentaccording to the present invention, showing the electrical isolation ofthe blade from the arbor and the mounting of the charge plates tocapacitively couple to the blade. Indicated in dash lines are a bracketfor mounting the charge plates, spacers between the charge plates andblade, and a brush contact mounted on the arbor block.

FIG. 4 is a magnified cross-sectional view take generally along the line4-4 in FIG. 3. For clarity, the mounting bracket indicated in FIG. 3 isnot shown.

FIG. 5 is a schematic cross-sectional view of another exemplaryembodiment according to the present invention in which the arbor iselectrically insulated from the arbor block and the charge plates arecapacitively coupled to the arbor.

FIG. 6 is a top plan view showing the isolation of, and capacitivecoupling to, an arbor on a contractor style table saw in accordance withanother embodiment of the present invention.

FIG. 7 is a cross-sectional view of the embodiment of FIG. 6 takengenerally along the central elongate axis of the arbor and viewing awayfrom the arbor block.

FIG. 8 is a top plan view showing an alternative assembly for couplingthe charge plates to the arbor of a contractor style table saw inaccordance with the present invention.

FIG. 9 is a cross-sectional view taken generally along the line 9-9 inFIG. 8.

FIG. 10 is a schematic side elevation of a further embodiment accordingto the present invention in the context of a band saw.

FIG. 11 is a magnified cross-sectional view taken generally along theline 11-11 in FIG. 10.

FIG. 12 is a side elevation of another embodiment according to thepresent invention in which contact with a guard is detected in thecontext of a radial arm saw.

DETAILED DESCRIPTION

A machine according to the present invention is shown schematically inFIG. 1 and indicated generally at 10. Machine 10 may be any of a varietyof different machines adapted for cutting workpieces, such as wood,including a table saw, miter saw (chop saw), radial arm saw, circularsaw, band saw, jointer, planer, etc. Machine 10 includes an operativestructure 12 having a cutting tool 14 and a motor assembly 16 adapted todrive the cutting tool. Machine 10 also includes a safety system 18configured to minimize the potential of a serious injury to a personusing machine 10. Safety system 18 is adapted to detect the occurrenceof one or more dangerous conditions during use of machine 10. If such adangerous condition is detected, safety system 18 is adapted to engageoperative structure 12 to limit any injury to the user caused by thedangerous condition.

Machine 10 also includes a suitable power source 20 to provide power tooperative structure 12 and safety system 18. Power source 20 may be anexternal power source such as line current, or an internal power sourcesuch as a battery. Alternatively, power source 20 may include acombination of both external and internal power sources. Furthermore,power source 20 may include two or more separate power sources, eachadapted to power different portions of machine 10.

It will be appreciated that operative structure 12 may take any one ofmany different forms, depending on the type of machine 10. For example,operative structure 12 may include a stationary housing configured tosupport motor assembly 16 in driving engagement with cutting tool 14.Alternatively, operative structure 12 may include a movable structureconfigured to carry cutting tool 14 between multiple operatingpositions. As a further alternative, operative structure 12 may includeone or more transport mechanisms adapted to convey a workpiece towardand/or away from cutting tool 14.

Motor assembly 16 includes one or more motors adapted to drive cuttingtool 14. The motors may be either directly or indirectly coupled to thecutting tool, and may also be adapted to drive workpiece transportmechanisms. Cutting tool 14 typically includes one or more blades orother suitable cutting implements that are adapted to cut or removeportions from the workpieces. The particular form of cutting tool 14will vary depending upon the various embodiments of machine 10. Forexample, in table saws, miter saws, circular saws and radial arm saws,cutting tool 14 will typically include one or more circular rotatingblades having a plurality of teeth disposed along the perimetrical edgeof the blade. For a jointer or planer, the cutting tool typicallyincludes a plurality of radially spaced-apart blades. For a band saw,the cutting tool includes an elongate, circuitous tooth-edged band.

Safety system 18 includes a detection subsystem 22, a reaction subsystem24 and a control subsystem 26. Control subsystem 26 may be adapted toreceive inputs from a variety of sources including detection subsystem22, reaction subsystem 24, operative structure 12 and motor assembly 16.The control subsystem may also include one or more sensors adapted tomonitor selected parameters of machine 10. In addition, controlsubsystem 26 typically includes one or more instruments operable by auser to control the machine. The control subsystem is configured tocontrol machine 10 in response to the inputs it receives.

Detection subsystem 22 is configured to detect one or more dangerous, ortriggering, conditions during use of machine 10. For example, thedetection subsystem may be configured to detect that a portion of theuser's body is dangerously close to, or in contact with, a portion ofcutting tool 14. As another example, the detection subsystem may beconfigured to detect the rapid movement of a workpiece due to kickbackby the cutting tool, as is described in U.S. Provisional PatentApplication Ser. No. 60/182,866, the disclosure of which is hereinincorporated by reference. In some embodiments, detection subsystem 22may inform control subsystem 26 of the dangerous condition, which thenactivates reaction subsystem 24. In other embodiments, the detectionsubsystem may be adapted to activate the reaction subsystem directly.

Once activated in response to a dangerous condition, reaction subsystem24 is configured to engage operative structure 12 quickly to preventserious injury to the user. It will be appreciated that the particularaction to be taken by reaction subsystem 24 will vary depending on thetype of machine 10 and/or the dangerous condition that is detected. Forexample, reaction subsystem 24 may be configured to do one or more ofthe following: stop the movement of cutting tool 14, disconnect motorassembly 16 from power source 20, place a barrier between the cuttingtool and the user, or retract the cutting tool from its operatingposition, etc. The reaction subsystem may be configured to take acombination of steps to protect the user from serious injury. Placementof a barrier between the cutting tool and teeth is described in moredetail in U.S. Provisional Patent Application Ser. No. 60/225,206,entitled “Cutting Tool Safety System,” filed Aug. 14, 2000 by SD3, LLC,the disclosure of which is herein incorporated by reference. Retractionof the cutting tool from its operating position is described in moredetail in U.S. Provisional Patent Application Ser. No. 60/225,089,entitled “Retraction System For Use In Power Equipment,” filed Aug. 14,2000 by SD3, LLC, the disclosure of which is herein incorporated byreference.

The configuration of reaction subsystem 24 typically will vary dependingon which action(s) are taken. In the exemplary embodiment depicted inFIG. 1, reaction subsystem 24 is configured to stop the movement ofcutting tool 14 and includes a brake mechanism 28, a biasing mechanism30, a restraining mechanism 32, and a release mechanism 34. Brakemechanism 28 is adapted to engage operative structure 12 under theurging of biasing mechanism 30. During normal operation of machine 10,restraining mechanism 32 holds the brake mechanism out of engagementwith the operative structure. However, upon receipt of an activationsignal by reaction subsystem 24, the brake mechanism is released fromthe restraining mechanism by release mechanism 34, whereupon, the brakemechanism quickly engages at least a portion of the operative structureto bring the cutting tool to a stop.

It will be appreciated by those of skill in the art that the exemplaryembodiment depicted in FIG. 1 and described above may be implemented ina variety of ways depending on the type and configuration of operativestructure 12. Turning attention to FIG. 2, one example of the manypossible implementations of safety system 18 is shown. System 18 isconfigured to engage an operative structure having a cutting tool in theform of a circular blade 40 mounted on a rotating shaft or arbor 42.Blade 40 includes a plurality of cutting teeth (not shown) disposedaround the outer edge of the blade. As described in more detail below,braking mechanism 28 is adapted to engage the teeth of blade 40 and stopthe rotation of the blade. U.S. Provisional Patent Application Ser. No.60/225,210, entitled “Translation Stop For Use In Power Equipment,”filed Aug. 14, 2000 by SD3, LLC, the disclosure of which is hereinincorporated by reference, describes other systems for stopping themovement of the cutting tool. U.S. Provisional Patent Application Ser.No. 60/225,058, entitled “Table Saw With Improved Safety System,” filedAug. 14, 2000 by SD3, LLC, and U.S. Provisional Patent Application Ser.No. 60/225,057, entitled “Miter Saw With Improved Safety System,” filedAug. 14, 2000 by SD3, LLC, the disclosures of which are hereinincorporated by reference, describe safety system 18 in the context ofparticular types of machines 10.

In the exemplary implementation, detection subsystem 22 is adapted todetect the dangerous condition of the user coming into contact withblade 40. The detection subsystem includes a sensor assembly, such ascontact detection plates 44 and 46, configured to detect any contactbetween the user's body and the blade. The detection subsystem isadapted to transmit a signal to control subsystem 26 when contactbetween the user and the blade is detected. Various exemplaryembodiments and implementations of detection subsystem 22 are describedin more detail in U.S. Provisional Patent Application Ser. No.60/225,200, entitled “Contact Detection System For Power Equipment,”filed Aug. 14, 2000 by SD3, LLC, the disclosure of which is hereinincorporated by reference.

Control subsystem 26 includes one or more instruments 48 that areoperable by a user to control the motion of blade 40. Instruments 48 mayinclude start/stop switches, speed controls, direction controls, etc.Control subsystem 26 also includes a logic controller 50 connected toreceive the user's inputs via instruments 48. Logic controller 50 isalso connected to receive a contact detection signal from detectionsubsystem 22. Further, the logic controller may be configured to receiveinputs from other sources (not shown) such as blade motion sensors,workpiece sensors, etc. In any event, the logic controller is configuredto control operative structure 12 in response to the user's inputsthrough instruments 48. However, upon receipt of a contact detectionsignal from detection subsystem 22, the logic controller overrides thecontrol inputs from the user and activates reaction subsystem 24 to stopthe motion of the blade. Various exemplary embodiments andimplementations of control subsystem 26 are described in more detail inU.S. Provisional Patent Application Ser. No. 60/225,059, entitled “LogicControl For Fast Acting Safety System,” filed Aug. 14, 2000 by SD3, LLC,and U.S. Provisional Patent Application Ser. No. 60/225,094, entitled“Motion Detecting System For Use In Safety System For Power Equipment,”filed Aug. 14, 2000 by SD3, LLC, the disclosures of which are hereinincorporated by reference.

In the exemplary implementation, brake mechanism 28 includes a pawl 60mounted adjacent the edge of blade 40 and selectively moveable to engageand grip the teeth of the blade. Pawl 60 may be constructed of anysuitable material adapted to engage and stop the blade. As one example,the pawl may be constructed of a relatively high strength thermoplasticmaterial such as polycarbonate, ultrahigh molecular weight polyethylene(UHMW) or Acrylonitrile Butadiene Styrene (ABS), etc., or a metal suchas aluminum, etc. It will be appreciated that the construction of pawl60 will vary depending on the configuration of blade 40. In any event,the pawl is urged into the blade by a biasing mechanism in the form of aspring 66. In the illustrative embodiment shown in FIG. 2, pawl 60 ispivoted into the teeth of blade 40. It should be understood that slidingor rotary movement of pawl 60 may also be used. The spring is adapted tourge pawl 60 into the teeth of the blade with sufficient force to gripthe blade and quickly bring it to a stop.

The pawl is held away from the edge of the blade by a restrainingmechanism in the form of a fusible member 70. The fusible member isconstructed of a suitable material adapted to restrain the pawl againstthe bias of spring 66, and also adapted to melt under a determinedelectrical current density. Examples of suitable materials for fusiblemember 70 include NiChrome wire, stainless steel wire, etc. The fusiblemember is connected between the pawl and a contact mount 72. Preferablyfusible member 70 holds the pawl relatively close to the edge of theblade to reduce the distance the pawl must travel to engage the blade.Positioning the pawl relatively close to the edge of the blade reducesthe time required for the pawl to engage and stop the blade. Typically,the pawl is held approximately 1/32-inch to ¼-inch from the edge of theblade by fusible member 70, however other pawl-to-blade spacings mayalso be used within the scope of the invention.

Pawl 60 is released from its unactuated, or cocked, position to engageblade 40 by a release mechanism in the form of a firing subsystem 76.The firing subsystem is coupled to contact mount 72, and is configuredto melt fusible member 70 by passing a surge of electrical currentthrough the fusible member. Firing subsystem 76 is coupled to logiccontroller 50 and activated by a signal from the logic controller. Whenthe logic controller receives a contact detection signal from detectionsubsystem 22, the logic controller sends an activation signal to firingsubsystem 76, which melts fusible member 70, thereby releasing the pawlto stop the blade. Various exemplary embodiments and implementations ofreaction subsystem 24 are described in more detail in U.S. ProvisionalPatent Application Ser. No. 60/225,056, entitled “Firing Subsystem ForUse In Fast Acting Safety System,” filed Aug. 14, 2000 by SD3, LLC, U.S.Provisional Patent Application Ser. No. 60/225,170, entitled“Spring-Biased Brake Mechanism for Power Equipment,” filed Aug. 14, 2000by SD3, LLC, and U.S. Provisional Patent Application Ser. No.60/225,169, entitled “Brake Mechanism For Power Equipment,” filed Aug.14, 2000 by SD3, LLC, the disclosures of which are herein incorporatedby reference.

It will be appreciated that activation of the brake mechanism willrequire the replacement of one or more portions of safety system 18. Forexample, pawl 60 and fusible member 70 typically must be replaced beforethe safety system is ready to be used again. Thus, it may be desirableto construct one or more portions of safety system 18 in a cartridgethat can be easily replaced. For example, in the exemplaryimplementation depicted in FIG. 2, safety system 18 includes areplaceable cartridge 80 having a housing 82. Pawl 60, spring 66,fusible member 70 and contact mount 72 are all mounted within housing82. Alternatively, other portions of safety system 18 may be mountedwithin the housing. In any event, after the reaction system has beenactivated, the safety system can be reset by replacing cartridge 80. Theportions of safety system 18 not mounted within the cartridge may bereplaced separately or reused as appropriate. Various exemplaryembodiments and implementations of a safety system using a replaceablecartridge are described in more detail in U.S. Provisional PatentApplication Ser. No. 60/225,201, entitled “Replaceable Brake MechanismFor Power Equipment,” filed Aug. 14, 2000 by SD3, LLC, and U.S.Provisional Patent Application Ser. No. 60/225,212, entitled “BrakePositioning System,” filed Aug. 14, 2000 by SD3, LLC, the disclosures ofwhich are herein incorporated by reference.

While one particular implementation of safety system 18 has beendescribed, it will be appreciated that many variations and modificationsare possible within the scope of the invention. Many such variations andmodifications are described in U.S. Provisional Patent Application Ser.Nos. 60/182,866 and 60/157,340, the disclosures of which are hereinincorporated by reference.

As mentioned above, contact detection plates 44 and 46 are used todetect contact between the user's body and cutting tool 14. It will beappreciated that detection subsystem 22 may employ any one or more of awide variety of methods for detecting contact between the blade and auser's body. In view of the relatively high response speed of electronicsignals and circuits, one suitable method includes using electricalcircuitry to detect an electronic connection between a user and thecutting tool. It has been found that the capacitance of a user's body,as measured through dry contact with a portion of the user's body, isapproximately 25-200 picofarads. The measured contact capacitance tendsto increase with increasing body size and with increased couplingbetween the user's body and an electrical ground.

As a result of the inherent capacitance of a user's body, when the usertouches cutting tool 14, the capacitance of the user's body iselectrically coupled to the inherent capacitance of the cutting tool,thereby creating an effective capacitance that is larger than theinherent capacitance of the cutting tool alone. Thus, detectionsubsystem 22 may be electrically coupled to measure the capacitance ofthe cutting tool, so that any substantial change in the measuredcapacitance would indicate contact between the user's body and thecutting tool.

The exemplary implementation depicted in FIG. 2 illustrates a detectionsubsystem 22 that is configured to detect contact between a user and thecutting tool through a capacitive coupling between the blade and plates44, 46. Detection system 22 includes suitable electrical circuitry(e.g., such as described in U.S. Provisional Patent Application Ser. No.60/225,200, entitled Contact Detection System for Power Equipment, filedAug. 14, 2000, by SD3, LLC) to transmit an input signal to plate 44, andto detect the input signal through plate 46. Plate 44 is mounted closeto, but spaced-apart from, blade 40. Plate 44 is capacitively coupled tothe saw blade by virtue of its size and placement parallel to andspaced-apart from the saw blade. Plate 46 is also mounted close to, butspaced-apart from, the saw blade to establish a second capacitivecoupling. It is within the scope of the present invention that thenumber, size and placement of charge plates may vary.

The effect of this arrangement is to form two capacitors in seriesthrough the blade, creating a capacitive shunt at the junction betweenthe capacitors. Plates 44 and 46 function as charge plates of thecapacitors. The input signal is capacitively coupled from charge plate44 onto blade 40, and then capacitively coupled from the blade to chargeplate 46. Any change in the capacitance of the blade changes the signalcoupled to charge plate 46.

When a user touches blade 40, the capacitance of the user's body createsa capacitive load on the blade. As a result, the size of the capacitiveshunt between the charge plates and the blade is increased, therebyreducing the charge that reaches plate 46. Thus, the magnitude of theinput signal passed through the blade to plate 46 decreases when a usertouches the blade. Detection subsystem 22 is configured to detect thischange in the input signal and transmit a contact detection signal tologic controller 50.

In some cases, there may be a significant amount of resistance at thecontact point of the user's dry skin and the blade. This resistance mayreduce the capacitive coupling of the user's body to the blade. However,when the teeth on the blade penetrate the outer layer of the user'sskin, the moisture inherent in the internal tissue of skin will tend todecrease the resistance of the skin/blade contact, thereby establishinga solid electrical connection. The sensitivity of detection subsystem 22can be adjusted as desired to recognize even slight changes in the inputsignal.

Generally speaking, the spacing of the charge plates from the blade isnot critical, and may vary depending on the charge plate area and thedesired capacitive coupling with the blade. However, it may be desirableto separate the plates from the blade by a distance selected to reducethe effect of deflections in the blade on the capacitance between theblade and the plates. For instance, if the blade is displaced 1/32 of aninch toward one of the plates by loads created during cuttingoperations, the capacitance to that plate is increased. Since thecapacitance is proportional to the area of the plate divided by thespacing, a relatively large spacing reduces the relative effect of agiven blade displacement. Distances in the range of approximately 1/32inch and approximately ½ inch have proven effective, although valuesoutside this range could be used under appropriate circumstances.

It will be appreciated that the charge plates may be positioned at anypoint adjacent one or both sides and/or the perimeter of the blade. Inthe exemplary embodiment, the plates are disposed relatively close tothe center of the blade. Since the deflection of the blade typically isat a minimum near the arbor upon which it is mounted, placing the chargeplates close to the arbor has the advantage of minimizing the effect ofblade deflection on the capacitive coupling between the plates and theblade. In various alternative embodiments, the outer edges of at leastone of the charge plates is radially spaced within 50%, 40%, 30%, 20% or10% of the blade's radius from the center of the blade.

The charge plates may be mounted within machine 10 in any suitablefashion known to those of skill in the art. For example, in theexemplary embodiment depicted in FIG. 3, operative structure 12 includesa pivotal arbor block 250 adapted to support arbor 42. The charge platesare mounted on a support member 251 (shown in dashed lines in FIG. 3),which is attached to arbor block 250. As a result, charge plates 44 and46 pivot with the arbor block, thereby maintaining their positionadjacent the blade. Alternatively, the charge plates may be mounted in astationary configuration.

In an alternative embodiment, at least one of the charge plates mayinclude one or more insulating spacers 252 mounted on the side of thecharge plate adjacent the blade, such as shown in FIGS. 3 and 4. Spacers252 act as physical barriers to prevent the blade from deflecting tooclose to the charge plate. This may be especially useful when thedistances between the charge plates and the blade are relatively small.The spacers may be constructed of any suitable electrically insulatingmaterial, including ceramic, glass, plastic, etc. In the exemplaryembodiment depicted in FIGS. 3 and 4, spacers 252 cover only a smallportion of the area between the charge plates and the blade. As aresult, the spacers have relatively little effect on the capacitancebetween the blade and the plate. Alternatively, the spacers may cover asubstantially larger portion, or even all of the space between thecharge plates and the blade. In this latter case, the spacer willfunction, at least partially, as the dielectric between the conductivesurfaces of the charge plates and the blade. Thus, the capacitancebetween the blade and the charge plates will depend on the dielectricconstant of the spacer.

In addition to the one or more spacers mounted between the charge platesand the blade, opposing spacers (not shown) may be mounted on the sideof the blade opposite the charge plates to prevent the blade fromdeflecting too far from the charge plates. Alternatively, one chargeplate may be mounted on the opposite side of the blade from the othercharge plate. Further, the spacers may be designed to slide on thesurface of the blade as it moves. Additionally, if the charge plates aremounted to move into and away from the side of the blade, andresiliently biased toward the blade, the charge plates and spaces willmove with any deflections of the blade, thereby maintaining contactbetween the spacers and blade even when the blade is deflected. Anadvantage of this arrangement is the close spacing that can beestablished and maintained, thereby reducing the size of the plates andmaintaining a constant capacitance between the charge plate and blade.

It will be appreciated that the size of charge plates 44 and 46 may alsovary. Typical plate areas are between 1 and 10 square inches, althoughmany different sizes may be used, including sizes outside of thistypical range. In the exemplary embodiment, the charge plate sizes areselected, in conjunction with charge plate spacing and dielectricmaterial, to provide a charge plate-to-blade capacitance that iscomparable (e.g., within an order of magnitude) with the capacitance ofthe human body. This configuration serves to improve the signal-to-noiseratio of the input signal detected by charge plate 46. Furthermore,charge plate 44 may be a different size than charge plate 46 and/or bespaced closer or farther apart from the blade to provide differentcapacitances. For example, it may be desirable to size drive chargeplate 44 larger than sense charge plate 46 to increase the coupling ofthe drive charge plate.

An example of a suitable charge plate material is copper-plated printedcircuit board, which is relatively rigid, flat and thin. Other examplesinclude any relatively electrically conductive material such as gold,aluminum, copper, steel, etc. The charge plates may take any shapesuitable for the particular clearances of machine 10. Where there arelarge grounded metal structures near the blade, a larger driving chargeplate 44 can be used to partially shield the blade from capacitivecoupling to the grounded structure. Although the larger plate also willhave increased capacitive coupling to the grounded structure, this doesnot interfere with the operation of detection subsystem 22 because thedetection subsystem is capable of driving much larger capacitance loadsthan are created under these circumstances.

It will be appreciated by those of skill in the art that blade 40 shouldbe insulated from electrical ground to allow the input signal to becapacitively coupled from charge plate 44 to charge plate 46. In theexemplary embodiment depicted in FIGS. 3 and 4, blade 40 is electricallyisolated from arbor 42 on which it rides, thus insulating the blade fromground and the remaining structure of the machine. There are a varietyof suitable arrangements for providing electrical insulation between theblade and the arbor, which may vary depending on the particularconfiguration of machine 10. For example, in the case of a ⅝-inch arborshaft 42, blade 40 can be formed with a one-inch diameter hole intowhich a 3/16-inch thick cylindrical plastic bushing 253 is fitted, suchas shown in FIGS. 3 and 4. Insulating washers 254 are disposed on eitherside of the blade to isolate the blade from the arbor flange 255 andarbor washer 256. The insulating washers should be thick enough thatonly negligible capacitance is created between the blade and thegrounded arbor flange and washer. A typical thickness is approximately⅛-inch, although 1/32-inch or less may be suitable depending on otherfactors. In addition, it is possible to construct some or all of thearbor components from non-conductive materials, such as ceramic, toreduce or eliminate the need for electrical isolation from the arbor.

An arbor nut 257 holds the entire blade assembly on arbor 42. Frictionestablished by tightening the arbor nut allows torque from the arbor tobe transmitted to the saw blade. It is preferable, although notessential, that the blade be able to slip slightly on the arbor in theevent of a sudden stop by the brake to reduce the mass that must bestopped and decrease the chance of damage to the blade, arbor, and/orother components in the drive system of the saw. Furthermore, it may bedesirable to construct the bushing from a material that is soft enoughto deform when the blade is stopped suddenly. For example, depending onthe type of braking system used, a substantial radial impact load may betransmitted to the arbor when the brake is actuated. A deformablebushing can be used to absorb some of this impact and reduce the chanceof damage to the arbor. In addition, proper positioning of the brake incombination with a deformable bushing may be employed to cause the bladeto move away from the user upon activation of the brake, as is discussedU.S. Provisional Application Ser. No. 60/225,089, entitled RetractionSystem for Use in Power Equipment, filed Aug. 14, 2000, by SD3, LLC.

It will be appreciated that the blade insulation assembly describedabove does not require special saw blades such as are described in U.S.Pat. No. 4,026,177. Indeed, arbor 42 may be sized to fit within aplastic bushing 253 received within a standard saw blade 40 having a⅝-inch diameter hole. Thus, an operator may use any standard blade onmachine 10.

As an alternative to insulating the blade from the arbor, the arborand/or part of its supporting framework may be electrically isolatedfrom ground. One benefit of this embodiment is that if the blade iselectrically connected to the arbor, then the arbor itself can be usedto capacitively couple the input signal from charge plate 44 to chargeplate 46. As a result, the charge plates are unlikely to interfere withinstallation and removal of the blade, and thus unlikely to be damagedor removed by a user. While the particular implementation of thisalternative embodiment will vary with the configuration of the cuttingtool, one exemplary implementation is depicted in FIG. 5.

As shown, blade 40 is mounted directly onto arbor 42. As in FIG. 4, theblade is secured to the arbor by arbor flange 255, arbor washer 256 andarbor nut 257. The arbor is supported for rotational movement relativeto an arbor block 250 by one or more bearings 258 mounted in the arborblock and spaced along the elongate axis of the arbor. However, bearings258 do not contact the arbor directly. Instead, electrically insulatingsleeves 259 are disposed between the arbor and the bearings. Arbor block250 is movable to allow the blade to be raised and lowered, as well asto be inclined for angled cuts. A motor (not shown) drives the arborthrough a belt 260 that loops over a pulley 261 on the end of the arboropposite the blade. The belt typically is non-conducting and thus doesnot electrically couple the arbor to ground.

Sleeves 259 may be constructed of any suitable material that isrelatively durable and non-conductive, including plastic, ceramic, etc.The sleeves may be configured to fit over a constant-diameter arbor asshown, or the arbor may be notched to receive the sleeves so that theouter diameter of the sleeves are flush with the outer diameter of thearbor. Furthermore, it will be appreciated that there are many otherarrangements for electrically insulating the arbor. As just a fewexamples, sleeves 259 may be disposed between bearings 258 and arborblock 250, or at least portions of the bearings may be constructed ofnon-conductive materials. For example, ceramic bearings may be used.Alternatively, larger portions of the arbor assembly may be isolatedfrom the rest of the saw.

In any event, charging plates 44 and 46 are disposed alongside, butslightly spaced from, the arbor. The charging plates typically areshaped and arranged relative to the arbor to ensure adequate capacitivecoupling. For example, the charging plates may be trough-shaped toconform to the cylindrical shape of the arbor, as illustrated in FIG. 5.Alternatively, the plates may be in the form of a ring or tube tocompletely surround axially-spaced portions of the arbor. The chargingplates typically are supported on arbor block 250, such as by mounts 262extending from the frame. This arrangement ensures that the chargingplates will move in tandem with the arbor when the position or angle ofthe blade is adjusted. The mounts usually will be configured toelectrically insulate the charging plates from the frame. The chargeplates can be positioned very close to the arbor because it does notdeflect during use like the blade, thereby allowing smaller chargeplates to be utilized.

Turning attention to FIGS. 6 and 7, an alternative arrangement forcapacitively coupling charge plates 44 and 46 to arbor 40 is shown. Thisarrangement has proven suitable for use with contractor style table sawswhich are available from a variety of manufacturers. Arbor block 250includes two spaced-apart, and generally parallel support members 263adapted to receive bearings 258 within central recesses 264.Electrically-insulating bushings 265 are disposed in the bearings andadapted to receive arbor 42. Each bushing 265 includes an outer lip orflange 266 which abuts the outer edges of the bearing. The bushings maybe constructed of ERTYLITE™ (PET-P), or any otherelectrically-insulating material adapted to support the arbor within thebearings.

Arbor flange 255 is integrally formed with arbor 42 and abuts againstthe flange of one of bushings 265. The opposite end of arbor 42 isthreaded to receive one or more locking nuts 267, which tighten againstthe flange of the other bushing 265 to retain arbor 42 within bearings258. Pulley 261 is mounted on the arbor adjacent locking nuts 267.

As shown in FIG. 7, bushings 265 completely insulate the arbor from thebearings and the arbor block. Alternatively, the bushings could beconfigured to fit between bearings 258 and support members 263. In anyevent, the arbor remains securely and symmetrically positioned to rotatefreely within the bearings.

Charge plates 44 and 46 take the form of electrically-conductive tubeshaving inner diameters larger that the diameter of arbor 42. Tubes 44,46 may be constructed of any suitable material such as brass tube,copper pipe, etc. It will be appreciated that the size of charge tubes44 and 46 may be selected to provide a desired capacitance with thearbor. Indeed, the size of the charge tubes may be different to providedifferent capacitances. For example, in the embodiment depicted in FIGS.6 and 7, charge tube 44 is longer than charge tube 46, thereby providinga higher capacitance between charge tube 44 and the arbor, than betweencharge tube 46 and the arbor. Alternatively, or additionally, the insidediameters of the charge tubes may be different to provide differentcapacitances due to different arbor-to-charge plate spacings.

Charge tubes 44 and 46 are received in an electrically-insulatingsupport housing or tube 268, having an inner diameter adapted to receivecharge tubes 44 and 46. Insulating tube 268 may be formed of anysuitable electrically-insulating material such as polycarbonate, nylon,PVC, etc. The insulating tube serves to prevent the charge tubes frombeing grounded by the arbor block, bearings, etc. Insulating tube 268 ispositioned around arbor 42 and received into inner apertures 269 insupport members 263. Inner apertures 269 are axially collinear witharbor 42. Thus, where charge tubes 44 and 46 are centrally positionedwithin the insulating tube, the inner diameters of the charge tubes areautomatically positioned by the insulating tube to be axially collinearor symmetrical with the arbor.

It will be appreciated that while the charge tubes and insulating tubein the exemplary embodiment are cylindrical, other shapes may also beused. For example, insulating tube 268 may have a rectangular outercross-section while maintaining its circular inner cross-section.Likewise, charge tubes 44 and 46 may have any suitable outercross-sectional shape to match the inner shape of the insulating tube.In any event, mounting the charge tubes to support members 263 ensuresthat the support tubes maintain the correct position about the arborregardless of the movement of arbor block 250.

In addition to electrically insulating and automatically positioning thecharge tubes, insulating tube 268 also serves to enclose and protect thecharge tubes from damage and debris. In the exemplary embodiment,insulating tube 268 defines a hole 270 positioned between charge tube 44and charge tube 46 to allow electrical cables (not shown) to be solderedor otherwise connected to the charge tubes to carry the signals to andfrom the detection circuitry of detector subsystem 22. Alternatively,two holes may be used, each positioned over one of the charge tubes.

Since the charge tubes should not come into contact with each other, thefit between the charge tubes and insulating tube is typically tightenough to frictionally prevent movement of the charge tubes along theaxis of the insulating tube. Alternatively, a bump or ring may be formedor positioned on the inner diameter of the insulating tube between thecharge tubes to prevent the charge tubes from coming into contact. As afurther alternative, hole 270 may be used to apply a caulk, glue, epoxy,or similar material between the charge tubes and insulating tube toprevent the charge tubes from moving. As another alternative, one ormore set-screws may be threaded through the insulating tube to bearagainst the charge tubes.

Turning attention now to FIGS. 8 and 9, an alternative embodiment of theinsulating tube and charge tubes for use with a contractor style saw isdepicted. Insulating tube 268 includes a hollow bore with outwardlybeveled ends to receive charge tubes 44 and 46. Each charge tube has aninner narrowed rim portion 271 to which an electrical cable (not shown)may be attached (e.g., by solder, etc.). The narrowness of rims 271allow the cables to be attached before the charge tubes are insertedinto the insulating tube. Typically, the cables are fed through hole270.

Insulating tube 268 also includes a recessed region 272 adapted toreceive a Hall Effect or similar sensor assembly 1000 for detectingblade/arbor rotation. Sensor 1000 is described in more detail in U.S.Provisional Patent Application Ser. No. 60/225,094, entitled MotionDetection System for Use in Safety System for Power Equipment, filedAug. 14, 2000, by SD3, LLC. The sensor is aligned over a hole 273 incharge tube 44 to sense the passage of a magnet disposed on the arbor(not shown). Alternatively, the sensor may be aligned over a hole 273 incharge plate 46. In some cases, such as where charge plates 44 and 46are identical, it may be desirable to place hole 273 in both chargeplates to reduce the number of different parts for manufacture.

While a few exemplary arrangements for capacitively coupling the chargeplates to the arbor have been described, it will be understood thatthere are many suitable arrangements and that the invention is notlimited to any particular one. For example, if there is insufficientroom between the bearings for the charge plates, one or both of thecharge plates may be positioned between the bearings and the pulley, oron the side of the pulley opposite the bearings.

It will appreciated that one or both of the charge plates may becapacitively coupled to other portions of operative structure 12 ratherthan blade 40 or arbor 42. For example, charge plates 44 and 46 may becoupled to an arbor block 250 which is electrically insulated from theremainder of the operative structure and machine 10. In such aconfiguration, the blade should be electrically coupled to the arborblock. Therefore, insulating bushings between the blade and arbor, orbetween the arbor and arbor block, should be omitted. As additionalexamples, the charge plates may be coupled to the bearings, pulley, etc.

It also will be appreciated that charge plates 44 and 46 may becapacitively coupled to other types of cutting tools, including thosewith a non-circular blade or cutter. For example, FIGS. 10 and 11 depictan exemplary embodiment in which the charge plates are capacitivelycoupled to the blade of a band saw 275. Typically, band saw 275 includesa main housing 276 enclosing a pair of vertically spaced-apart wheels277. The perimeter of each wheel 277 is coated or covered in ahigh-friction material such as rubber, etc. A relatively thin,continuous loop blade 40 tightly encircles both wheels. A workpiece iscut by passing it toward blade 40 in a cutting zone 278 between wheels277. An upper blade-guide assembly 279 and a lower blade-guide assembly280 maintain the revolving blade in a stable path within cutting zone278. The workpiece is passed toward the blade on a table 281, whichforms the bottom of the cutting zone.

The blade should be electrically insulated from the main housing, whichusually is grounded. Thus, blade-guide assemblies 279 and 280, which mayinclude ball-bearing guides and/or friction pads, etc., are constructedto electrically insulate the blade from the main housing. In addition,the high-friction coating on wheels 277 electrically insulates the bladefrom the wheels. Alternatively, the wheels may be constructed ofelectrically non-conductive material.

Charge plates 44 and 46 may be arranged in a variety of ways dependingon the application and the space constraints within the main housing.Two possible arrangements are illustrated in FIG. 10. In the firstarrangement, charge plates 44 and 46 are disposed closely adjacent theblade as it rides along one of the wheels 277. The charge plates may beformed in an arc to match the perimeter of the wheel and maintain aconstant spacing with the blade. This arrangement has the advantage ofeasily maintaining a constant blade-to-charge plate spacing since theblade is held in a constant path against the perimeter of the wheel. Thecharge plates may be connected to the main housing via a non-conductivemount to maintain electrical insulation from the housing.

Another of the many possible arrangements for the charge plates includesa charge plate block 282 which is configured to extend along the bladeas it travels between wheels 277. As can best be seen in the detail viewof FIG. 11, the charge plate block includes charge plates 44 and 46. Inthe depicted implementation, the charge plate block has a substantiallyC-shaped cross-section sized to fit around the sides and back edge(i.e., non-toothed edge) of the blade. The charge plate block is mountedon main housing 276 and resiliently biased, such as by one or moresprings 283, toward the moving blade. Since blade 40 may tend to move ordeflect slightly in its path, springs 283 ensure that the charge plateblock is able to move along with blade. Charge plate block 282 typicallyis made of a durable, electrically non-conductive material such asceramic, plastic, etc. Charge plates 44 and 46 are disposed on or withinthe charge plate block. Although the charge plates are illustrated asbeing disposed on opposite sides of blade 40, the charge plates mayalternatively be on the same side of the blade. The self-aligningconfiguration of the charge plate block ensures that the blade-to-chargeplate spacing is substantially constant despite the motion of the blade.

In addition to band saws, the charge plates may be capacitively coupledto machines such as jointers, planers, etc., which have cylindricalcutter heads. The cutter heads typically are mounted to rotate about anarbor. Thus, charge plates 44 and 46 may be capacitively coupled to thearbor as described above, or to a flat end of the cutter head, etc.

While one exemplary system and method for detecting contact between theuser's body and the blade is described herein, many other systems andmethods are available and within the scope of the invention. Forexample, the detection system may sense the resistance of the human bodyupon contact between the user's body and the blade. As shown in FIG. 3,the sensor assembly of detection subsystem 22 may include a brushcontact 284 or similar sensor to make direct electrical contact with theblade. Brush contact 284 may be mounted, for example, on arbor block250. Typically, the blade and brush contact are electrically isolatedfrom the arbor block. Alternatively, the brush contact may be configuredto directly couple to the arbor or another portion of operativestructure 12 as described above in connection with charge plates 44 and46. In any event, contact between the user's body and blade wouldfunction as a switch to form a conductive path detectable by suitablecircuitry in detection subsystem 22 and/or control subsystem 26. As afurther alternative, brush contact 284 may be used to detect acapacitive rather than conductive load upon the blade. As a furtheralternative, the detection subsystem sensor assembly may be configuredto detect contact by optical, magnetic, or other non-electrical means.

As an alternative to detecting contact between the user and the blade,detection subsystem 22 may be configured to detect proximity of theuser's body to the blade by detecting contact between the user's bodyand a guard adjacent the blade. If the guard is positioned so that theuser's body must contact the guard before contacting the blade, then theblade may be stopped before the user comes into contact with the blade.It will be appreciated that this alternative detection subsystem may beimplemented in a variety of different configurations and for any type ofmachine 10. As one example, FIG. 12 shows an exemplary embodiment foruse on a radial arm saw 286.

Typically, radial arm saw 286 includes a horizontal base 287, a verticalsupport column 288 extending upward from base 287, and a guide arm 289which extends from column 288 vertically spaced above base 287. Acarriage 290 is slidably coupled to the underside of guide arm 289. Thebottom end of carriage 290 is connected to a saw housing 291 and motorassembly 16, allowing blade 40 to be pulled across the base to cutworkpieces (not shown) supported on the base. A guard member 292, suchas those known in the art, is positioned on at least one side of blade40. Guard member 292 is disposed relative to the blade so that anyportion of the user's body approaching the blade will first strikeagainst the guard member. Typically, guard member 292 is movably coupledto housing 291 to maintain its blade-shielding position as the bladepasses over the workpiece.

The guard member is electrically insulated from housing 291 butelectrically coupled to the detection subsystem (not shown). Thus, anycontact between the user's body and the guard member is detected. Thedetection subsystem may be conductively coupled to the guard member byany suitable means (not shown) such as electrical cable, etc.Alternatively, the detection subsystem may be capacitively coupled tothe guard member by one or more charge plates disposed adjacent theguard member such as described above.

As described above, the present invention provides a reliable, effectiveand fast-acting system for preventing serious injuries to operators ofpower cutting machinery. While a few specific embodiments of safetysystem 18 and machine 10 have been described above, those of skill inthe art will appreciate that the present invention may be adapted innumerous ways for use in a wide variety of applications. Therefore, itwill be understood that all such adaptations and applications are withinthe scope of the invention.

It is believed that the disclosure set forth above encompasses multipledistinct inventions with independent utility. While each of theseinventions has been disclosed in its preferred form, the specificembodiments thereof as disclosed and illustrated herein are not to beconsidered in a limiting sense as numerous variations are possible. Thesubject matter of the inventions includes all novel and non-obviouscombinations and subcombinations of the various elements, features,functions and/or properties disclosed herein. No single feature,function, element or property of the disclosed embodiments is essentialto all of the disclosed inventions. Similarly, where the claims recite“a” or “a first” element or the equivalent thereof, such claims shouldbe understood to include incorporation of one or more such elements,neither requiring nor excluding two or more such elements.

It is believed that the following claims particularly point out certaincombinations and subcombinations that are directed to one of thedisclosed inventions and are novel and non-obvious. Inventions embodiedin other combinations and subcombinations of features, functions,elements and/or properties may be claimed through amendment of thepresent claims or presentation of new claims in this or a relatedapplication. Such amended or new claims, whether they are directed to adifferent invention or directed to the same invention, whetherdifferent, broader, narrower or equal in scope to the original claims,are also regarded as included within the subject matter of theinventions of the present disclosure.

1. A method of detecting a dangerous condition between a person and awoodworking machine, where the woodworking machine includes a cuttingtool electrically coupled to an arbor, and where the cutting tool andarbor are electrically isolated, the method comprising: imparting anelectrical signal to the cutting tool through a capacitive coupling thatincludes two spaced-apart conductors, where at least a portion of thesurface of the arbor is one of the conductors; and monitoring theelectrical signal for changes indicative of the dangerous condition. 2.The method of claim 1 where the dangerous condition is contact betweenthe person and the cutting tool.
 3. The method of claim 1 where thedangerous condition is proximity between the person and the cuttingtool.
 4. The method of claim 1 where one of the two spaced-apartconductors is a charge plate positioned adjacent the surface of thearbor, and where the limitation of imparting an electrical signal to thecutting tool comprises inducing an electrical charge on the arbor byimparting an electrical charge to the charge plate.
 5. The method ofclaim 1 where the limitation of monitoring the electrical signal forchanges comprises using the electrical signal imparted to the cuttingtool to induce an electrical charge on a sense electrode positionedadjacent the surface of the arbor, and analyzing the electrical chargeon the sense electrode for predetermined characteristics.
 6. Awoodworking machine comprising: a motor; an electrically isolated arborconfigured to be driven by the motor; a cutting tool coupled to thearbor; an excitation system adapted to generate an electrical signal; acapacitive coupling adapted to capacitively couple the excitation systemto the arbor to transfer at least a portion of the electrical signal tothe cutting tool, where the capacitive coupling includes a conductiveplate substantially encircling the arbor; and an insulator substantiallyencircling the conductive plate.
 7. The woodworking machine of claim 6further comprising an opening in the insulator so that the excitationsystem can electrically couple to the conductive plate.
 8. Thewoodworking machine of claim 6 further comprising a detection systemconfigured to detect a dangerous condition between a person and thecutting tool and a second conductive plate adjacent the arbor.
 9. Thewoodworking machine of claim 8 where the second conductive platesubstantially encircles the arbor, and where the insulator substantiallyencircles the second conductive plate.
 10. The woodworking machine ofclaim 9 further comprising an opening in the insulator so that thedetection system can electrically couple to the second conductive plate.11. The woodworking machine of claim 9 further comprising a firstopening in the insulator so that the excitation system can electricallycouple to the conductive plate and a second opening in the insulator sothat the detection system can electrically couple to the secondconductive plate.
 12. The woodworking machine of claim 6 furthercomprising an arbor block adapted to support the arbor, and where thearbor block also supports the insulator.
 13. The woodworking machine ofclaim 12 where the arbor block includes two spaced-apart support membersand the insulator is held by the two spaced-apart support members. 14.The woodworking machine of claim 12 where the insulator supports theconductive plate.
 15. A band saw comprising: a motor; at least twospaced-apart wheels, where at least one wheel is driven by the motor; aband blade extending around the wheels, where the band blade iselectrically isolated; signal generation circuitry adapted to generatean electrical signal; a capacitive coupling adapted to capacitivelycouple the signal generation circuitry to the band blade to transfer atleast a portion of the electrical signal to the band blade; anddetection circuitry configured to monitor the electrical signal on theband blade for changes indicative of a dangerous condition between aperson and the band blade and to trigger some action to mitigate thedangerous condition when the changes indicative of the dangerouscondition are detected.
 16. The band saw of claim 15 further comprisinga second capacitive coupling adapted to capacitively couple thedetection circuitry to the band blade so that the detection circuitrycan monitor the electrical signal on the band blade through the secondcapacitive coupling.
 17. The band saw of claim 15 where the capacitivecoupling includes two spaced-apart conductors, and where the blade isone of the two conductors and a charge plate adjacent the blade is theother of the two conductors.
 18. The band saw of claim 17 where thecharge plate is adjacent a portion of the band blade extending aroundone of the wheels.
 19. The band saw of claim 17 where the charge plateis adjacent a portion of the band blade extending between the wheels.20. The band saw of claim 17 further comprising a second capacitivecoupling adapted to capacitively couple the detection circuitry to theband blade so that the detection circuitry can monitor the electricalsignal on the band blade through the second capacitive coupling, wherethe second capacitive coupling includes two spaced-apart conductors, andwhere the blade is one of the two conductors and a charge plate adjacentthe blade is the other of the two conductors.
 21. A woodworking machinecomprising: a motor; a cutting tool driven by the motor, where thecutting tool is electrically isolated; an excitation system adapted togenerate an electrical signal; a capacitive coupling adapted tocapacitively couple the excitation system to the cutting tool totransfer at least a portion of the electrical signal to the cuttingtool, where the capacitive coupling includes a conductive plate adjacentthe cutting tool; and an insulating spacer intermediate the cutting tooland conductive plate.
 22. The woodworking machine of claim 21 where thecutting tool has two sides and the conductive plate and insulatingspacer are adjacent one side of the cutting tool, and further comprisingat least one additional spacer adjacent the other side of the cuttingtool.
 23. The woodworking machine of claim 21 where the insulatingspacer is a dielectric.
 24. A woodworking machine comprising: a motor; acutting tool driven by the motor, where the cutting tool is electricallyisolated; an excitation system adapted to generate an electrical signal;means for capacitively-coupling the excitation system to the cuttingtool to transfer at least a portion of the electrical signal to thecutting tool; and means for positioning the capacitive coupling means inan operable configuration in the woodworking machine.
 25. Thewoodworking machine of claim 24 where the means for capacitivelycoupling the excitation system to the cutting tool includes at least onecharge plate, and where the means for positioning the capacitivecoupling means in an operable configuration includes at least onesupport member.